xe2x80x9cThermoplastic vulcanizatesxe2x80x9d or xe2x80x9cTPVsxe2x80x9d (also referred to in the past as xe2x80x9cthermoplastic elastomersxe2x80x9d or xe2x80x9cTPEsxe2x80x9d) are made by dynamic vulcanization of a blend of an olefin rubber and a crystalline polyolefin (xe2x80x9cPOxe2x80x9d) which becomes the continuous phase in which microscopic rubber particles are held. A processable TPV is formulated in which polyethylene (xe2x80x9cPExe2x80x9d) constitutes a major proportion by weight of the continuous PO phase (referred to herein as a xe2x80x9cPE-rich TPVxe2x80x9d), with melt viscosity reducers used in a defined range. TPVs formulated with a specified melt reducer in the stated range may contain PE in an amount greater than 25% by weight, based on all the polymer (PO and rubber) in the TPV.
The term xe2x80x9celastomerxe2x80x9d is used in the broad sense, in that the cured blend is processable as a TPV, and is re-processable, unlike a thermoset resin. By xe2x80x9cprocessablexe2x80x9d is meant that a dynamically vulcanized blend can be thermoformed, typically injection molded, extruded, vacuum-formed or blow-molded in a commercially available machine. Such extruders and injection molding machines for TPVs provide internal mixing at a temperature in the range from about 180xc2x0 C. to 240xc2x0 C. with a residence time less than 5 min, preferably in the range from 30 sec to 2 min. In practice, an attempt to make a useful and marketable TPV by substituting PE for polypropylene (xe2x80x9cPPxe2x80x9d) present in an amount greater than 25% based on the weight of polymer (PO and EPDM) present, in a randomly chosen EPDM results in a blend which is not processable. In processable TPVs which are xe2x80x9cself-curedxe2x80x9d and not physical blends, their combination of desirable elastic and thermoplastic properties depends on the respective amounts of xe2x80x9chardxe2x80x9d and xe2x80x9csoftxe2x80x9d phases provided by each component, and the properties of each component. In most cases, the prior art fails to recognize the unusually high melt viscosity of PE-based TPVs and routinely disclose that PP in PP/EPDM TPVs may be substituted with PE or any other polyolefin without the benefit of an enabling illustrative example.
Commercially available TPVs generally consist of micron-sized (1-10 xcexcm) crosslinked EPDM rubber particles embedded in a continuous phase mainly of PP having various crystallinities. EPDM is a copolymer of ethylene, propylene, and a diene providing a cure site monomer, most commonly ethylidenenorbornene (xe2x80x9cENBxe2x80x9d). Such TPVs are produced by the dynamic vulcanization of blends of EPDM rubber in molten PP, the rubber being selectively cured during intense mixing. (see xe2x80x9cThermoplastic Elastomersxe2x80x9d by G. Holden et al, ed. Chap 7, Hansen Publishers, 2nd ed., 1996).
For PP/EPDM TPVs, an increase in blend melt viscosity during TPV formation results presumably from the increased rubber/plastic contact area generated by the micron-sized particles in a continuous plastic phase. The viscous drag of the molten plastic over the rubber particles is a major contributor to melt viscosity of the TPV, with additional contributions due to deformation within and interactions between the rubber particles. Though melt viscosity of PP/EPDM TPVs allows their processability, a PE-rich TPV is not xe2x80x9cprocessablexe2x80x9d because of its unusually high melt viscosity.
In most known, usable TPVs, PP is the continuous hard phase and the EPDM is the soft phase present as discrete particles. In the novel TPV, PE is in the continuous xe2x80x9chardxe2x80x9d phase, and the xe2x80x9csoftxe2x80x9d phase is chosen from (i) a copolymer of ethylene-propylene-5-vinyl-2-norbornene, an EPDM rubber containing pendent vinyl unsaturation (hereafter either is referred to as xe2x80x9cEP(VNB)xe2x80x9d or xe2x80x9cpEPDMxe2x80x9d to connote the particular olefinic rubbers), and (ii) butyl rubber having a pendent vinyl cure site (referred to as xe2x80x9cpButylRxe2x80x9d) By varying the ratios of the components including the amount of processing oil, within limits beyond which the TPV is unusable, one is expected to be able to provide desired hardness/softness, oil and temperature resistance, oxidation resistance, and processability, inter alia.
In U.S. Pat. Nos. 3,957,919 and 4,059,654 to Von Bodungen, et al. the beneficial effect of PE in the TPV is secured when PE is present in an amount greater than 15% but not more than 25%, with the remainder of 70% to less than 85% by weight divided between the EPDM interpolymer and the monoolefin polymer in the ratio of 90-10 parts by weight of EPDM polymers to 10-90 parts by weight of monoolefin polymers. The PE component may include copolymers of ethylene containing 10% or less copolymerized xcex1-olefins having from 3-16 carbon atoms.
But the ""654 disclosure teaches that within the ambit of the proviso with respect to ratios, any PO may be added to a PE and PP combination in any EPDM and worked with any free radical generating agent to provide TPVs with acceptable compression set. This broad disclosure of any EPDM interpolymer reads on a vast array of EPDMs including ethylene-propylene-5-vinyl-2-norbornene; and, of any PO, reads on a vast array including poly-1-butene, and copolymers of ethylene-co-butene and ethylene-co-propene-1-butene, which are peculiar in that they have a melt viscosity lower than that of PP. It is believed that, unlike other poly-C3-C16-olefins, the common characteristic which makes the aforementioned polymers and some amorphous polymers useful as melt viscosity reducers in a PE-rich TPV is their peculiar morphology in the rubber/plastic contact area, and the resulting low viscous drag of the molten plastic over the rubber particles. The melt viscosity reducer appears to maintain itself as a separate phase in each of the phases of the vulcanizate, making the TPV processable.
EP(VNB) or pEPDM and pButylR are readily blended in the molten state in any proportions with PE. Such high compatibility of molten PE with these rubbers is greater than that of a blend in which PP is substituted for PE. However, when such a substantially xe2x80x9cPE onlyxe2x80x9d blend is vulcanized, the TPV has too high a viscosity to be processable; that is, when measured in an automatic capillary rheometer (xe2x80x9cACRxe2x80x9d), the viscosity is above 8000 Poise at 200xc2x0 C. Such high viscosity is too high to allow the components of the TPV to be processable in commercially available equipment. The high viscosity is attributable to the compatibility of PE with EPDM rubber. A TPV of PP/EP(VNB) or PP/pButylR in which either PP or the rubber is present in a larger amount than the other, is deemed processable; but a TPV of PE/EP(VNB), or PE/pButylR in which the rubber is present in a larger amount than the PE, or vice versa, defies extrusion or fabrication (referred to as having xe2x80x9cpoor fabricabilityxe2x80x9d) in such equipment.
Though the disclosure of the ""654 patent teaches that all TPVs containing high density PE in an amount greater than 15% by weight of the total EPDM-PO-PE but in which the PE component does not exceed 25% by weight, are ideally suited for use in the manufacture of flexible hose, EP(VNB) substituted for EPDM is not. Neither is a blend of EP(VNB) with 25% PE processable when cured with a silane curing agent. (see Examples 32, 33 and 34 in Table 8 herebelow).
Because trouble-free processsability in commercially available injection molding machines and extruders is tied to melt viscosity, a blend of rubber and PO only, that is, without any processing aids including oil, is required to have a melt viscosity in the range from about 200 P (Poise) to 4000 P. Above 4000 P the processability diminishes progressively and at 8000 P a blend is deemed unprocessable. At about 8000 P the melt viscosity of a TPV, as measured in a Monsanto ACR Model No. 3501, at 204xc2x0 C. (400xc2x0 F.) and 118 kPa constant stress is so high that there is no transfer, or an insufficient amount of transfer of polymer from the melting zone in the tube, to trip a switch in the tube at the end of 4 min, thus starting the measurement.
Blends having a melt viscosity in the range from 4000 P to 8000 P are made processable by adding conventional processing aids.
The Problem
To benefit from the properties of a TPV containing a major proportion of PE relative to polypropylene (xe2x80x9cPPxe2x80x9d), most preferably PE only, in the continuous plastic phase of the TPV, it is desired to use a vulcanizable blend of EPDM, polyolefin(s), and processing aids including melt viscosity reducers, which is xe2x80x9cprocessablexe2x80x9d to produce a TPV with desirable properties and in which TPV the processing oil does not bleed out. The result sought for is a fabricatable TPV in which the ratio of PE to EPDM is in the range from more than 1:3 to about 1:0.6, by weight, on an oil-free basis, based on total polymer present (defined herein as PO and EPDM only, excluding polymeric melt viscosity reducer); in particular, where the PO includes a PO other than PE, typically PP, PE itself is to be present in more than 25% by weight, based on total polymer present. The prior art failed to recognize that, for such PE-rich TPVs of commercial interest, a blend of components in the aforespecifed range becomes unprocessable in a one-step process. In other words, it is not feasible to process such TPVs in available commercial extruders and injection molding machines to make a marketable product. The problem was to find a combination of rubber and melt viscosity reducer (additive with specifically defined characteristics) which allows a PE-rich TPV containing a PE plastic phase, as specified, to be extruded, injection molded, or otherwise fabricated in a manner analogous to a TPV of PP/EPDM, e.g. those commercially available as Santoprene(copyright) brand TPVs, without substantially compromising the properties of the finished article of PE-rich TPV.
Solutions to date
Additives including processing aids, compatibilizers, plasticizers, extenders and melt viscosity reducers have, to date, been unsatisfactory in a PE-rich TPV because so large an amount of the additive was required to provide processability that either the properties of the TPV were compromised, or the additive exuded from the fabricated article over a period of less than one year. No acceptable solution has been provided to date.
Numerous prior art references teach using a PO and EPDM to form a TPV but limit their examples to the use of PP or PP containing up to 20% PE. Such references assumed that processing a blend of EPDM/olefin in which PE was substituted for the PP should provide no serious difficulty because PE, and HDPE in particular, is readily blended with a wide spectrum of cross-linkable rubbers before the blend is cured. As is well known, high ratios of PO, whether PP or PE, relative to EPDM, in the range above 6:1; and high ratios of EPDM to PO in the range above 6:1, are easily processable while curing because their ACR viscosities are lower than about 5000 Poise.
U.S. Pat. Nos. 4,130,534 and 4,130,535 to Coran et al disclose dynamically vulcanized thermoplastic elastomer compositions comprising butyl rubber and PO, and olefin rubber and PO, respectively. The addition of plasticizers and aromatic, naphthenic and paraffinic extender oils to the blend is suggested. No details are given regarding the choice or suitability of any particular class or type of plasticizers. It is well known that different rubbers are compatible with certain types of plasticizers and that not all plasticizers are suitable with all rubbers.
In U.S. Pat. No. 5,290,886, Ellul teaches that low molecular weight ( less than 10,000) organic esters and alkyl ether esters plasticize a blend of crystalline PO homopolymer or copolymer, and, an olefinic rubber, each present in the range from 10% to 90% of vulcanizate, provided the plasticizer is compatible with both phases, the PO and the rubber. These oils and waxes have a solubility parameter below 19.5. A method for calculating the solubility parameter of an oil or wax is disclosed in xe2x80x9cProperties of Polymersxe2x80x9d, Ch. 7, by D. W. vanKrevelen, Elsevier Press, Amsterdam, 1990. Moreover, the requirement for a plasticizer is that it be miscible in both phases, hard and soft; a melt viscosity reducer is not substantially miscible in both phases. Still further, the plasticizer used therein is a critical component to lower the Tg of the blend, and not related to providing processability. This is evident from the TPV made in Example 5 where all the plasticizer was added after dynamic vulcanization of a PP (219.1 parts), EPDM (100 parts) was completed. The TPV included active black (19.28 parts), clay (40 parts), curatives (10.5 parts). (see col 6 of ""886). A comparable PE/EPDM formulation will not come through the barrel of the Monsanto ACR.
The easy processability of the ""886 composition was predicated upon using a low molecular weight ester plasticizer in a major amount by weight of PP relative to the EPDM, and the PP may contain as much as 20% PE. The plasticizer provided the TPV with improved low temperature properties. Since the processability of the PP-rich blend is not related to the low temperature properties of the TPV but to the viscosity of the blend after curing, it is evident that processability was not a concern with the Ellul composition. To illustrate, in Example 5, the ACR viscosity was low because there is more plasticizer than EPDM; and more than twice as much isotactic PP as there is EPDM. The limitation on the amount of PE is dictated by the poor processability of the PP/EPDM vulcanizatexe2x80x94the PP may contain no more than 20% by weight (wt %) PE. Such vulcanizates are sensitive to ultra-violet (UV) light and thermal oxidation which sensitivity would be greatly diminished if PE was substituted for the PP. Moreover substituting PE for PP tends to yield elastomers which are tougher, stronger and softer, that is, have a lower Durometer reading. However, the temperature at which the EPDM/PE may be used will be lower than the PP/EPDM vulcanizates taught in the ""886 patent.
To evaluate the perceived advantages of using PE, processability is specified as a function of ACR viscosity measured under specified conditions representative of the mean processing conditions typically encountered in commercial processing equipment; and, processability of a PE-rich/EPDM blend with various additives is explored. The priority was to find an additive which remained as a separate phase in both phases, the PE and the rubber. Though the ""886 patent teaches that a low molecular weight plasticizer will improve low temperature properties of a PO/rubber vulcanizate, the patent does not teach the effect of such esters, or any other additives on melt viscosity; and the patent is silent as to what criteria might lead to the choice of a reducer which would lower melt viscosity. Since the problem addressed in the ""886 invention was to find a TPV with better low temperature properties, the emphasis was to identify those compounds which gave better low-temperature properties without regard to the processability of the composition with the plasticizer. Accordingly, all samples were prepared in a laboratory Brabender-Plasticorder (as they have been for the illustrative examples in this invention) without regard to the time or energy requirements, and being interested only in the low temperature properties of the TPV, measured viscosity only for the aforesaid composition (219.1 parts of PP and 100 parts of EPDM). There is no other reference to the viscosity of the mixture, for any reason.
It has been discovered that in combination with certain melt viscosity reducers, a blend of xe2x80x9cpEPDMxe2x80x9d or xe2x80x9cEP(VNB)xe2x80x9d with more than 25% by weight PE, based on all polymer present but excluding any polymeric melt viscosity reducer, is a processable blend, as defined. The pEPDM is most preferably present in a major amount by weight relative to the olefin continuous phase, and if PP is in the olefin phase, PE is present in a major amount by weight relative to PP in the cured TTV. A substantially fully cured vulcanizate of the blend is referred to as xe2x80x9cPE/pEPDMxe2x80x9d or xe2x80x9cPE/EP(VNB)xe2x80x9d. Preferably the ratio of PE to pEPDM in the blend is in the range from more than 1:3 and up to about 1:0.6, by weight, based on total polymer present, the PE always being present in an amount greater than 25% by weight, based on total polymer present, and the pEPDM interpolymer consisting essentially of ethylene in a major proportion relative to another polymonoolefin containing from 3-5 carbon atoms, and from 0.5% to 5% by weight VNB. This blend, containing from about 50 phr but less than about 200 phr of processing oil including an oil specifically designated as a melt viscosity reducer, is preferably processed in presently available commercial processing equipment by using an inert melt viscosity reducer which is compatible with the pEPDM or pButylR and PE phases without being substantially miscible in either. Such a reducer is chosen from (a) specific polyolefins which, as melt viscosity reducers, do not behave as other polyolefins of C2-C4 olefins in an EPDM blend, (b) melt viscosity reducing oils or waxes, and (c) a block copolymer of a fatty acid and polyethylene oxide having a Tg below 150xc2x0 C., most preferably, poly(12-hydroxystearic acid)-b-polyethylene glycol-b-poly(12-hydroxystearic acid). A specific melt viscosity reducing polyolefin is chosen from (i) isotactic poly(1-butene), ethylene-co-1-butene, and propene-co-1-butene having a MW in the range from 20,000 to about 900,000, preferably from 500,000 to 800,000, and (ii) ethylene-co-propene-co-1-butene having a MW in the range from 20,000 to about 900,000, preferably from 30,000 to 60,000. Melt viscosity reducing oils have a solubility parameter in the range from 19.5 to 21.5, relatively low molecular weight in the range from about 300 to about 2000, and have a high hydrocarbon content but a low level of polar organic groups. Such oils and waxes include epoxidized vegetable oils, and ethoxylated allylphenols; epoxidized soybean oil, ethoxylated castor oil, hydrogenated castor oil and ethoxylated nonylphenol. These criteria are believed to allow a specified oil to maintain itself as a separate phase in each of the phases of the vulcanizate, namely the PE phase and the rubber phase. By xe2x80x9csubstantially fully curedxe2x80x9d is meant that less than 3% rubber is extractable in cyclohexane at 23xc2x0 C.
It is therefore a general object of this invention to provide a composition comprising (a) a processable mixture of (i) an olefinic rubber selected from the group consisting of an ethylene-propylene-VNB rubber and a butyl rubber with a pendent vinyl cure site, and (ii) substantially crystalline PE and olefinic rubber present in a ratio ranging from more than 1:3 up to about 1:0.6, and, (b) from about 5 to about 50 parts, total, of one or more specified melt viscosity reducers per hundred parts of olefinic rubber and PE combined. The specified melt viscosity reducers are also effective to varying degrees with PE-rich TPVs produced from butyl rubber and PE where a minor portion relative to the PE may be PP.
It is a specific object of this invention to provide a processable PE-rich blend of a pEPDM and PE in combination with one or more of the aforespecified melt viscosity reducers, in which pEPDM, ethylene is present in the range from about 50 to 70% by weight, VNB is present in the range from about 0.5 to 5% by weight, the remainder being propylene.